1. Field of the Invention
The present invention relates to a four-wheel-drive vehicle in which a four-wheel drive state can be maintained by driving the main drive wheels with an internal combustion engine or other main drive source, and driving the subordinate drive wheels with an electric motor in an appropriate manner, and is particularly suitable for the engagement control of a clutch disposed between the electric motor and the subordinate drive wheels.
2. Background Information
An example of a driving force control apparatus for a batteryless four-wheel drive vehicle is disclosed in Japanese Laid-Open Patent Application No. 11-243608. In the four-wheel-drive vehicle of this publication, the front wheels are driven by an internal combustion engine, and the rear wheels are driven by an electric motor during a four-wheel drive. The output torque of the electric motor is commonly transferred to the rear wheel axle via a clutch and a reduction gear. In this apparatus, the clutch is engaged after the rotational speed of the rear wheel axle and the rotational speed of the electric motor are synchronized in order to prevent shocks from being generated by the clutch engagement during the transition to the four-wheel drive state.
The clutch is commonly connected to make a transition to a four-wheel drive state, and the subordinate drive wheels are brought into the drive state when the accelerator pedal is depressed and it is detected that the speed differential between the front and rear wheels is at or above a prescribed level, i.e., that the main drive wheels are experiencing an acceleration slippage that is at or above a prescribed level.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved vehicle driving force control apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.